An immunogenetic analysis of Friend virus leukemia is being performed. Friend virus induces erythroleukemia and profoundly immunosuppresses genetically susceptible mice. We detected a gene function (Fv-3) which regulates susceptibility of mice to immunosuppression in vivo and in vitro. The virus acts by "activating" T suppressor cells which mediate the suppression. Marrow dependent (M) cells regulate the T suppressor cells. We aim to determine if Fv-3 is an antigen restricted to T suppressor cells. M cells also function as natural killer (NK) cells capable of lysing tumor cells. We aim to determine if Fv-2 is an antigen expressed on Friend virus transformed cells. We detected "interfering cell" in lymphoid tissues which is responsible for the H-2 compatibility requirements for T suppressor cell:mitogen responsive cell interactions. We aim to fully characterize this cell which is thymus-dependent.